The UMTS Turbo Code and an Efficient Decoder Implementation Suitable for Software-Defined Radios

Due to their near Shannon-capacity performance, turbo codes have received a considerable amount of attention since their introduction [1]. They are particularly attractive for cellular communication systems and have been included in the specifications for both the WCDMA (UMTS) and cdma2000 third-generation cellular standards. At this time, the reasons for the superior performance of turbo codes [2,3] and the associated decoding algorithm [4,5] are, for the most part, understood. In addition, several textbooks [6–8] and tutorial papers [9–11] are now available to provide the interested reader with an understanding of the theoretical underpinnings of turbo codes. The purpose of this paper is neither to explain the phenomenal performance of turbo codes nor to rigorously derive the decoding algorithm. Rather, the purpose is to clearly explain an efficient decoding algorithm suitable for immediate implementation in a software radio receiver. In order to provide a concrete example, the discussion is limited to the turbo code used by the Universal Mobile Telecommunications System (UMTS) specification, as standardized by the Third-Generation Partnership Project (3GPP) [12]. The decoding algorithm is based on the log-MAP algorithm [13], although many parts of the algorithm have been simplified without any loss in performance. In particular, the branch metrics used in the proposed algorithm are much simpler to compute, and the amount of storage is reduced by 12.5% by an appropriate normalization process. Some critical implementation issues are discussed, in particular the The UMTS Turbo Code and an Efficient Decoder Implementation Suitable for Software-Defined Radios